Process of treating surfaces of shaped articles of polyacetals with camphorsulfonic acid



United States Patent of Germany No Drawing. Filed Oct. 20, 1964, Ser.No. 405,234

Claims priority, application Germany, Oct. 25, 1963, F 41,084 3 Claims.(Cl. 156-2) It is :known :to use high molecular weight polyacetals asthermoplasts for the manufacture of shaped articles, such as tubes,profiles and injection moulded material. Owing to the good resistance tochemicals of polyacetals, however, the shaped articles cannot be bondedor lacquered with satisfactory strength. Therefore, various methods havebeen proposed to treat the surfaces of shaped articles of polyacetals.

Experiments to roughen the surface mechanically with the aid of grindingcloth and analogous grinding agents, or to subject the shaped articlesto an electronic pretreatment under reduced pressure, or to treat thesurface with an oxidizing flame did not result in a suflicientimprovement of the bonding strength of adhesives and lacquers.

Better results are obtained when the shaped articles are immersed in asolution containing an oxidizing acid. It is known, for example, toroughen the surface of shaped articles of polyoxymethylene by dippingthem for minutes into a solution of about 20 C, of 11 grams of potassiumbichromate in 400 parts of sulfuric acid of 95-98% strength and 14 partsof water so that it can be well wetted with water and thus bonded andlacquered. However, the process has considerable drawbacks in that itcan only be applied when the area of the shaped article to be treated isfree from frozen in or forced stresses (latent stress) since otherwiseit breaks under the action of the acid. Shaped articles can be producedvery seldom without stress and it is, therefore, necessary to subjectthe shaped articles to a heat treatment before treating them with theoxidizing acid. Moreover, it is rather difficult to treat limited areasof the surface.

According to another proposal shaped articles which are under stress canbe treated with a mixture containing p-toluene-sulfonic acid. For thispurpose the shaped article is dipped for 10-30 seconds into an agitatedmixture of 75120 C. consisting of 0.3% by weight of ptoluene-sulfonicacid, 96.2% by weight of perchloroethylene, 3.0% by weight of dioxaneand 0.5% by weight of kieselguhr, then placed for 1 minute in a furnacehaving a temperature of 120 C., rinced with water and dried in the air.After said treatment the surface of the articles can be bonded andlacquered. However, it is not uniformly roughened because the amount ofp-toluenesulfonic acid adhering to the surface after dipping isnon-uniformly distributed on the said surface in spite of the additionof kieselguhr, especially when articles of complicated shape areconcerned. With this process, too, it is difiicult to limit thepretreatment to definite areas of the surface.

The present invention provides a process for improving the property ofbeing bonded and lacquered of surfaces of shaped articles of polyacetalswhich comprises contacting the dry surface for a period of time rangingfrom 15 seconds to 30 minutes at 70150 C. with dry pulverulentcamphor-sulfonic acid. After the removal of the loosely adhering acid,for example by blowing it off with air, the surface of the shapedarticle of polyacetal has a uniformly mat appearance, it can be wettedwith water Patented June 7, 1966 and has a good bonding strength foradhesives and lacquers. It is especially advantageous that according tothe process of the invention shaped articles with strong latent stresscan be treated without heat treatment or other pretreatment. Moreover,parts of the surface can be covered and thus readily protected againstthe action of the acid. Panels, for example can be treated on one sideonly or parts can be protected by covering them with adhesive foils.

In special cases, for example when larger amounts of shaped articles areto be treated in continuous manner, it may be of advantage to treat themwith dry pulverulent camphor-sulfonic acid fluidized with a hot carriergas, for example hot air.

Owing to the fact that camphor-sulfonic acid does practically notdecompose at high temperatures, it can repeatedly be used for thesurface treatment of shaped articles of polyacetals. It is possible toeffect a dry treatment of the surfaces of plastics also withp-toluenesulfonic acid. However, it is disadvantageous that. the latteracid decomposes during the first use and must be washed off with water.

According to the process of the invention any kind of shaped articles ofpolyacetals can be treated. By polyacetals there are to be understoodhomopolymers of formaldehyde or trioxane and the copolymers thereof withcyclic ethers of the formula in which R andR stand for hydrogen or lowmolecular weight alkyl radicals, or low molecular weight,halogensubstituted alkyl radicals, and R represents a methylene, anoxymethylene, or a low molecular weight alkylorhalogen-a1kyl-substituted methylene or a low molecular weight alkylorhalogen-alkyl-substituted oxymethylene radical and n is one of thenumbers 0,' 1, 2, and 3. Especially suitable are polyacetals such aspolyoxy-methylenes the terminal groups of which may have been esterifiedor etherified.

The following examples serve-to illustrate the invention but they arenot intended to limit it thereto.

Example 1 Pulverulent camphor-sulfonic acid was spread on panels ofpolyoxymethylene by means of a fine-meshed sieve in a manner such thatthe surface was uniformly covered. The covered panels were then heatedfor 3 minutes at 120 C. After having removed the powder and washed otfthe adhering traces of acid with water, the panels dried in air had amat surface. They could be bonded with one another and with panels ofwood, iron and other plastic materials by means ofpolyisocyanate/polyester adhesive. The adhesive joints had a tensileshear strength of kp./cm., whereas untreated polyoxymethylene panelscould not be bonded with the said adhesive.

Example 2 the basis of polyisocyanate/castor oil. The lacquer could notbe peeled off when the surface was scratched.

Example 3 2 mm. thick sheets (5 x 2.5 cm.) of polyoxymethylene were bentto an extent such that both ends could be inserted in a U-shaped profile1.3 cm. wide. The bent sheets were treated as described in Examples 1|and 2 Without stress crazing being observed. A pretreatment of the bentsheets with sulfuric acid bichromate solution was not possible becausein contact with the acid stress crazing occurred at once.

We claim:

1. Process for improving the property of being bonded and lacquered ofsurfaces of shaped articles of polyacetals, which comprises contactingthe dry surface for a period of time ranging from seconds to minutes atto C. with dry pulverulent camphor-sulfonic acid.

2. The process of claim 1, which comprises treating the shaped articlesby dipping them in dry camphor-sulfonic acid fluidized with a hotcarrier gas.

3. The process of claim 1, which comprises treating shaped articlescontaining high molecular weight polyoxymethylene.

References Cited by the Examiner UNITED STATES PATENTS 3,069,287 12/1962 Hudson 11747 OTHER REFERENCES Bruner et al.: Modern Plastics,December 1961, pp. 97, 98, 101, 106, 185, and 192 relied on.

WILLIAM D. MARTIN, Primary Examiner.

M. KATZ, Assistant Examiner.

1. PROCESS FOR IMPROVING THE PROPERTY OF BEING BONDED AND LACQUERED OF SURFACES OF SHAPED ARTICLES OF POLYACETALS, WHICH COMPRISES CONTACTING THE DRY SURFACE FOR A PERIOD OF TIME RANGING FROM 15 SECONDS TO 30 MINUTES AT 70 TO 150*C. WITH DRY PULVERULENT CAMPHOR-SULFONIC ACID. 